This invention relates to refrigeration devices for operating at cryogenic temperatures, and, more particularly, to orifice pulse tube cryocoolers. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
Significant effort has been expended to develop efficient and reliable cryocoolers for many applications where cryogenic temperatures are needed. Initially, development was driven by defense needs for effective optical sensors in the IR spectrum. Commercial electronics companies have recently funded cryocooler development in order to access the capabilities of cryogenic CMOS circuitry and the potential capabilities of high temperature superconductors operating at liquid nitrogen (as opposed to liquid helium) temperatures. Many designs and products for both Stirling engine coolers and orifice pulse tube coolers have been developed and applied. In general, Stirling devices have been found to be more efficient (a factor of 2 is quoted in some literature) than orifice pulse tube cooler devices. However, the orifice pulse tube approach has better reliability due to fewer moving parts (in some designs, no moving parts). In many applications, the vibrations from a Stirling device are unacceptable and the orifice pulse tube is the preferred approach.
The cryogenic/liquefied industrial gases industry consists of the liquefaction/separation of air, the liquefaction of hydrogen, the liquefaction of helium, and the liquefaction of petroleum gases. The majority of liquefied gas product is formed in large-scale plants where energy consumption and power efficiency are important concerns. While the overall cycle from raw material to final, purified liquid varies dramatically across this set of gases (and from plant to plant in some gases), the cycle invariably includes a final expansion cooling process to form the liquefied gas.
Air liquefaction plants use an isentropic expansion step for the final cooling. In this approach, the pre-cooled, compressed gas is expanded through a turbine. By performing work in passing through the turbine, a high degree of cooling of the gas is ensured. The turbine drives a compressor that compresses the overhead gas (that part of the gas flow that did not condense during expansion) prior to re-injecting it into the liquefaction flow stream. Most research on improving gas liquefaction technology appears to focus on improving the design of the turbo-expanders to achieve better work extraction and improved condensation.
In the liquefied natural gas market, a few establishments use refrigeration machines to cool and condense the product gas. These refrigeration-based systems use proprietary mixtures of light hydrocarbons (propane, ethylene, methane) whose refrigeration cycle is intricately integrated with the cooling of the natural gas (from which these refrigerator working fluids are originally obtained). It is possible that these refrigerants could be replaced by cryocoolers provided the overall process obtains adequate condensation efficiency.
Applications of cryocooling to superconductors fall into two groups: cooling of electronic components incorporating superconductors and cooling of large scale superconductor windings used as electromagnets in such devices as MRIs, NMR, particle accelerators, and power generators. Applications for these components include the power industry, the medical/diagnostic industry, the analytical instrument industry, and the high energy physics industry. Essentially all existing devices use a passive cryogen supply system in which the superconductor is supplied with cryogen from a reservoir. The reservoir must be periodically resupplied by a liquified gas supply company.
For purposes of comparison to a pulse tube refrigerator (PTR), a Stirling refrigerator may be regarded as consisting of several aligned components: hot compressor piston, hot heat exchanger, regenerator, cold heat exchanger, and cold expander piston. A conventional PTR 10 shown in FIG. 1A operates similarly, except that the cold expander piston is replaced with four stationary components: pulse tube 24 with heat exchanger 26, orifice 12, and reservoir 28. Hot compressor piston 14, hot heat exchanger 16, regenerator 22, and cold heat exchanger 18 complete PTR 10. Stirling refrigerators are more efficient than PTR refrigerators for three reasons. First, work is absorbed and dissipated into waste heat in orifice 12 of PTR 10, whereas work is efficiently recovered at the cold expander piston of the Stirling refrigerator and delivered back to the hot compressor piston. Second, the effective thermal conductance of pulse tube 24 often puts a greater thermal load on cold heat exchanger 18 than does the heat generated by friction and other losses at the cold expander piston in the Stirling refrigerator. Third, control of the time-phase relationship between mass flow and pressure is easily accomplished in the Stirling refrigerator, but is limited in the PTR. In the Stirling refrigerator, mass flow phase leads the pressure phase at the hot heat exchanger and lags pressure phase at the cold heat exchanger. In conventional PTRs the mass flow phase lags the pressure phase at both the hot heat exchanger 16 and cold heat exchanger 18, as shown in FIG. 1B.
This occurs because reservoir 16 is typically large enough to comprise a negligible impedance, and orifice 12 is a resistive impedance, so that the mass flow and pressure are in phase at heat exchanger 26, as seen in FIG. 1B. The compressibility of the gas in pulse tube 24 causes the mass flow phase at cold heat exchanger 18 to lead that at heat exchanger 26; similarly, the compressibility of the gas in regenerator 22 causes the mass flow phase at hot heat exchanger 16 to lead that at cold heat exchanger 18.
K. Kanao et al., "A Miniature Pulse Tube Refrigerator for Temperatures below 100 K," 34 Cryogenics, ICEC Supplement, pp.167-169 (1994), reports that a PTR orifice can be replaced with a small tube connecting the pulse tube with the reservoir, where the flow impedance between the pulse tube and the reservoir is adjusted by selecting tubes of differing diameter and length to optimize PTR performance. Zhu et al., "Phase Shift Effect of the Long Neck Tube for the Pulse Tube Refrigerator," Proceedings of the 9.sup.th International Cyrocoolers Conference held June 1996 (Preprint--to be published), further discusses the effect of a long neck tube inserted between the pulse tube hot end and the reservoir. Replacing the orifice with a long neck tube is taught to produce a pressure-mass flow phase shift that can be changed by changing the diameter and length of the long neck tube. It will be appreciated that these references discuss only the effect of replacing conventional orifice 12 with a long neck tube connecting pulse tube 24 with reservoir 28. While PTR performance optimization is discussed, there is no discussion or analysis relating to the optimization. In accordance with the present invention, the effect of acoustic impedance on PTR performance is analyzed and a variable acoustic impedance is introduced to optimize PTR performance.
Accordingly, it is an object of the present invention to control the phase relationship between mass flow and pressure to improve the operating efficiency of a PTR.
Yet another object of the present invention is to recover power from the orifice end of the PTR.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.